Planar optical waveguides are typically used in optical communications to implement devices such as directional coupler switches, phase modulators and interferometric amplitude modulators. In such applications, planar waveguides are typically coupled to optical fibers at their input and output facets. Among the potential significant sources of power losses are those deriving from a mismatch between the fundamental modes of the planar waveguide and the optical fibers connected at these facets. Without special care, power losses at each facet can be very high, as much as 75 percent or greater.
In order to achieve low coupling losses between an optical waveguide and a fiber, the distribution of electromagnetic radiation at the facets of the waveguide should be roughly equivalent to the distribution provided by the optical fiber coupled at the facet. It has proven extremely difficult, however, to manufacture planar waveguide devices in which the distribution of electromagnetic radiation is roughly equivalent to that of the optical fibers commonly in use today. In particular, planar optical waveguides are made up of layers so as to have a rectangular geometry at their facets, whereas optical fibers are cylindrical in shape and have a circular or elliptical geometry at their facets.
In an optical fiber, the usual arrangement is that guiding and confinement of the optical fields are produced by changes in the refractive index that are distributed in a circularly symmetric or elliptical manner with respect to the cross-section of the fiber. The majority of optical fiber now used in telecommunication systems, particularly in long-distance systems, is monomode with a core of higher refractive index of the order of 15 microns or less wide, and a cladding of lower refractive index whose outer diameter is of the order of 125 microns. These fibers are used to transmit radiation of a wavelength in the range of 0.8 to 1.65 microns, the radiation propagating along the fiber in a single transverse mode. The beam spot generally has dimensions in the range of 5 to 15 microns and the cross-section of the beam is circularly symmetric or elliptical as a result of the distribution of refractive index changes in the fiber.
In contrast to the geometry of an optical fiber, a planar waveguide device is generally based on a slab of material in which changes in refractive index are more easily produced along flat interfaces rather than in curved distributions. For instance, a semiconductor planar waveguide device may be manufactured in the form of epitaxially grown layers of material on a substrate. Changes in refractive index can then most easily be produced in each of two perpendicular directions. First, changes can be produced at the interfaces between the layers of material by using material of different refractive indices. Second, changes in the perpendicular direction can be produced by making steps in the layers of materials, for instance by etching using a mask. The steps may then either be left exposed to air, which has a low refractive index compared to semi-conductor materials, or buried in suitable material of preselected refractive index. In general, these planar waveguides can be classified into a number of different basic types, including rib guides, strip-loaded guides, buried-channel guides and slab guides of various kinds.
In order to provide a low-loss coupling between the optical fiber and planar waveguide, it is known to modify the modal shape at the output of the fiber in an attempt to match the shape to the modal shape of the waveguide. For example, the output of an optical fiber can be focused somewhat through tapers and spherical lenses, but control over its fundamental mode is generally limited. Most of the available options for tailoring the modal shape of the fiber involve changing the radial distance scale while leaving the field pattern essentially circular and thus still poorly matched to the elliptically shaped modes typically associated with waveguides. Although it may be possible to alter the modal shapes of the fibers by using special lenses, the small sizes involved create experimental and production difficulties--e.g., alignment of special lenses used to interface the waveguide.
Some success has been achieved in altering the shape of the waveguide modes for a certain class of waveguides for the purpose of matching the modal shape of the waveguide to an optical fiber. In U.S. Pat. No. 4,776,655 to Robertson et al., a type of rib waveguide is disclosed that has values for its compositional and structural parameters that provide a modal shape which is approximately matched to the modal shape of a mating optical fiber.
For a rib waveguide of the type illustrated by Robertson et al., the guiding zone is well defined in the lateral direction by the presence of a material of lower refractive index, typically air, on either side of the rib. Therefore, in a lateral direction, there is a large change in refractive index that provides strong optical confinement. According to the Robertson et al. patent, the modal shape in the waveguide can be adjusted to provide an elliptical shape that approximates the circular shape of the optical fiber by providing a small change in the refractive index between the core and cladding on the order of 0.01 to 0.0001. This range of differences in the refractive indices provides a measure of control of the modal shape in the direction perpendicular to the layers of the waveguide. The Robertson et al. patent also provides ranges of values for the structure of the rib with respect to its height and width to further sculpt the shape of the modal structure propagated by the waveguide.
In contrast to the rib guide described in the Robertson et al. patent, which laterally confines a light beam by means of etching the rib into the guiding layer, a raised-rib waveguide is a less common guide that provides a rib etched into an upper cladding layer grown over the guiding layer. Both types of waveguides utilize the rib to laterally confine the light beam as it propagates along the guiding layer. Conventional rib waveguides are the most common and have been studied to various degrees. A raised-rib waveguide, however, has been examined in less detail and is less widespread.
A raised-rib waveguide shares many of the functional characteristics of the conventional rib waveguide, but it offers some advantages that make it an attractive alternative to the conventional rib guide. In both the conventional and raised-rib guide, some light is inevitably lost by scattering due to roughness at the air-guide interface. Such roughness develops when the upper layer is partially etched away to create the rib. By including an upper cladding, however, the raised-rib guide provides a dielectric buffer between the air and the core, thereby reducing the modal fields at the air-waveguide interface and also reducing the scattering at the surface of the waveguide. At the same time, the weaker fields at the surface of the raised-rib guide also limit to some extent the ability of the rib to confine the modes laterally.
In a raised-rib waveguide, the relationship between the values of each of the structural and compositional parameters and the shape of the modal structure is complex since they strongly affect both lateral and perpendicular confinement. In the rib waveguide of the Robertson et al. patent, the lateral confinement of the modal structure is well defined because the guiding region is bounded, laterally, by material of lower refractive index. In the raised-rib waveguide, lateral confinement is relatively weak because the guiding region is not bounded laterally by material of low refractive index and changes such as those suggested in the Robertson et al. patent in order to shape the modal structure provided by the waveguide have different effects in both the perpendicular and lateral dimensions of the guide.